General Discussion

The cold bismuth solution, weakly acid with nitric acid, is treated with an excess of potassium iodide when BiI₃ and some K[BiI₄] are formed:

Bi(NO₃)₃ + 3KI = BiI₃ + 3KNO₃

BiI₃ + KI = K[BiI₄]

Upon dilution and boiling, bismuth oxyiodide is formed, and is weighed as such after suitable drying.

BiI₃ + H₂O = BiOI + 2HI

(black)

K[BiI₄] + H₂O = BiOI + KI + 2HI

(yellow)

A large excess of potassium iodide should be avoided, since the complex salt is not so readily hydrolysed as the tri-iodide. This is an excellent method, because the oxyiodide is precipitated in a form which is very convenient for filtration and weighing.

Procedure

The cold bismuth nitrate solution, containing 0.1-0.15 g of Bi, must be slightly acid with nitric acid (see Note), and occupy a volume of about 20 mL. Add finely powdered solid potassium iodide, slowly and with stirring, until the supernatant liquid above the black precipitate of bismuth tri-iodide is just coloured yellow (due to K[BiI₄]). Dilute to 200mL with boiling water, and boil for a few minutes. The black tri-iodide is converted into the copper-coloured precipitate of the oxyiodide. The supernatant liquid should be colourless; if this is yellow, a further 100 mL of water should be added, and the boiling continued until colourless. Add a few drops of methyl orange indicator, and then sodium acetate solution (25 g L^-1) from a burette until the solution is neutral. Filter off the precipitate through a weighed sintered-glass or porcelain filtering crucible, wash with hot water, and dry at 105-110 °C to constant weight. Weigh as BiOI.

Note: Chloride and bromide should be absent. If the solution is strongly acid with nitric acid, it should be evaporated to dryness on the water bath, and the residue dissolved in a little dilute nitric acid.